Individual selective rework of defective BGA solder balls

ABSTRACT

A method for reworking a ball grid array (BGA) of solder balls including one or more defective solder balls on an electronic component workpiece using a single-ball extractor/placer apparatus having a heatable capillary tube pickup head optionally augmented with vacuum suction. A defective solder ball is identified, extracted by the pickup head and disposed of. A nondefective solder ball is picked up by the pickup head, positioned on the vacated attachment site, and thermally softened for attachment to the workpiece. Flux may be first applied to the replacement solder ball or to the vacated attachment site. The extractor/placer apparatus may be automated to locate, extract and replace defective balls for completion of a fully operable BGA.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to semiconductor device manufacturing.More particularly, the instant invention pertains to methods andapparatus for removal and replacement of individual, defective solderballs on an electronic component.

2. State of the Art

Integrated circuit semiconductor devices (IC's) are small electroniccircuits formed on the surface of a wafer or other substrate ofsemiconductor material such as silicon, gallium arsenide or indiumphosphide. The IC devices are fabricated simultaneously in large numbersin wafer form in an array over the active surface of the wafer andtested by a probe to determine electronic characteristics applicable tothe intended use of the IC's. The wafer is then subdivided or“singulated” into discrete IC chips or dice, and then further tested,assembled with other components and packaged for customer use throughvarious wellknown individual die IC testing and packaging techniques,including leadframe packaging (conventional and leads-over-chip, orLOC), Chip-On-Board (COB) packaging, and flip-chip packaging. Dependingupon the relative die and wafer sizes, each wafer is singulated into atleast a few dozen dice, as many as several hundred dice, or even as manyas several thousand discrete dice when large (such as 30 cm) wafers areemployed.

Mechanical and electrical interconnection of discrete semiconductor dicewith a carrier substrate, such as a printed circuit board (PCB), isoften accomplished with an array of solder balls or bumps projectingfrom the active surface of the semiconductor die, these externalinterconnection elements usually having a spherical or near-sphericalshape, although other shapes are known. Such a package comprises theaforementioned flip-chip package, so called because the semiconductordie or chip is mounted active surface down over the carrier substrate,supported by the solder balls or bumps. State of the art flip-chippackages may comprise so called “chip-scale” packages, wherein thelateral footprint of the package is the same as, or minimally largerthan, the lateral dimension of the semiconductor die itself.

Several methods for forming solder balls or bumps on a workpiece arewell known. In the early art, a preformed solder ball was manuallyplaced on a semiconductor die using a forceps or pincer. In laterdevelopments, preformed balls have been deposited on bond pads on asemiconductor substrate using a single-ball mounting head or full ballgrid array (BGA) mounting head, using vacuum to retain the ball(s) onthe head prior to placement on the workpiece. Flux is applied either tothe pads or the balls prior to ball placement.

Mounting heads configured to simultaneously apply all balls of a BGA fora semiconductor die are preferred because of savings in labor costs. Inthe current state of the art, ball grid arrays may even be formed on allof the dice of a full wafer prior to semiconductor die singulationtherefrom. Thus, upwards of 10,000 balls may be placed on a wafer priorto the singulation process.

Currently, solder balls may be formed on a workpiece by processes ofevaporation, electroplating, stencil printing and serial methods. Eachof these processes has particular limitations.

In one version, the solder balls are temporarily fastened to the bondpads of a die by heating to a softening temperature and/or bycompression during application. The die with the array of balls placedthereon is then subjected to a thermal “reflow” step to return the ballsto substantially spherical shape and then cooled to harden the balls.

In another version, a solder paste preform of any shape may be placed ona metallized bond pad and melted to form a globular or “ball” shapefixedly attached to the bond pad. The ball shape is affected by surfacetension of the solder and solder-wettable bond pad or cup-shapedreceptacle on the semiconductor die. Alternately, other non-solderwettable passivation materials surrounding a bond pad or receptacle maybe utilized to assist in preventing undue solder spread or collapse intoadjacent balls (resulting in short-circuits) or damage to the diesurface surrounding the balls.

Numerous problems may occur in forming a BGA of a large number of ballson a semiconductor die, wafer or other workpiece, and in the subsequentattachment of the BGA to a carrier substrate. The following discussionpertains to merely a few of such problems.

Where a perforated multiple ball vacuum pickup head is used tosimultaneously place all of the BGA's solder balls on a workpiece, acommon complaint is that one or more ball-retaining holes is not filled,resulting in workpiece bond pads or other terminal areas devoid ofsolder balls. In U.S. Pat. No. 4,871,110 to Fukasawa et al., a proposedsolution is to provide a second perforated plate above the pickup headto retain the balls therein while sweeping extra balls across thesurface to ensure that all holes are filled.

U.S. Pat. No. 5,284,287 to Wilson et al. denotes two problems: nonpickupof solder balls by a multi-ball pickup tool and loss of solder ballswhile contacting them with flux in a flux bath. In the Wilson patent,solder balls are only partially submerged in the flux and never touchthe bottom of the flux bath.

U.S. Pat. No. 5,467,913 to Namekawa et al. discloses a solder ballattachment apparatus in which flux is separately applied to each pad onthe semiconductor substrate prior to attaching the solder balls.

U.S. Pat. No. 5,680,984 to Sakemi is directed to a solder ballattachment method using a multi-ball head. The solder balls on the headare dipped in flux prior to placement and reflow. The Sakemi patentnotes that when a solder ball is lost in the flux bath, it is recoveredin a groove by wiping with a squeegee. No mention is made of what isdone to correct the pickup head having an incomplete array of solderballs.

Single-ball pickup heads are known in the art for the purpose of placingsolder balls on conductive pads of a workpiece. An example of such isdescribed in U.S. Pat. No. 5,506,385 to Murakami et al. in which vacuumis used to hold a solder ball on a tubular pickup head. While sometimesuseful where the number of solder balls on the workpiece is few, its usein forming multi-ball BGA's is contraindicated, being generally veryslow, labor-intensive, and expensive. In the Murakami et al. reference,the apparatus uses a spring-biased head which holds a single solderball, picked up from one of a series of containers holding balls ofdiffering sizes. Flux is applied to each pad, followed by application ofa solder ball and thermal reflow resulting from a laser beam focused onthe ball.

U.S. Pat. No. 5,695,667 to Eguchi et al. describes an apparatus forforming a BGA of solder balls on a workpiece. A first multi-ball pickuphead is utilized to apply the majority of balls to the workpiece. Acamera is used to detect empty pads (i.e., having balls missingtherefrom). A second, single-ball pickup head is used to fill in emptyspaces, and the workpiece is heated in a furnace to reflow all of thesolder balls.

Solder balls installed on the workpiece may be defective in variousways. For example, a ball may be undersize (and, thus, not be adequatelyconnected to both a die and the carrier substrate during bonding), orthe ball may be oversize (and prevent other adjacent balls from beingadequately bonded to the carrier substrate or spread to contact anadjacent ball). The solder ball may also be irregular in shape,resulting in defective bonding. In addition, a solder ball may contain asurface inclusion which prevents or inhibits proper reflow. A solderball may also be misaligned with its pad, resulting in defective contacttherewith. In the current state of the art, such defects are simplydealt with by removing all of the solder balls on a given workpiece andstarting over. The “repair” is thus very time-consuming,material-consuming and expensive. None of the above-indicated referencesappear to recognize or address such problems.

The current emphasis on increased miniaturization and sophistication ofintegrated circuits has resulted in a continuing reduction in devicedimensions, ball diameter and ball spacing (pitch), and increasingnumbers of balls in a BGA. As the ball size is decreased, the relativenonuniformity in ball dimensions has been observed to increase.Likewise, as pitch becomes finer, a much greater precision in ballplacement is required, inasmuch as lateral ball-to-ball contact must beavoided. The increased numbers of balls required to be transferred toeach semiconductor die enhances the opportunity for missed solder balls,extra solder balls, and solder balls outside of the acceptable ranges ofsize or shape. Thus, the problems indicated hereinabove are exacerbatedby the ongoing commercial race to further miniaturize and densifysemiconductor devices and the like.

The BGA format has been used with discrete conductive elements otherthan solder balls, such as conductive epoxy bumps, conductor-filledepoxy bumps and the like, each of which presents its own set ofproblems. However, solder balls, such as are formed of tin/lead alloycompositions, remain the most widely used conductive elements in BGAconstructions. This is primarily because solder is relativelyinexpensive and the technologies for ball formation and placement arewell developed.

The use of flip-chip technology with solder balls has numerousadvantages for interconnection, as compared to conventional leadframetype packages. Flip-chip provides improved electrical performance forhigh frequency applications such as mainframes and computerworkstations. In addition, easier thermal management and reducedsusceptibility to electromagnetic interference (EMI) and radiofrequencyinterference (RFI) emissions are inherent. Furthermore, small solderballs may be densely packed in a BGA array within the footprint of asemiconductor die, which approach conserves surface area (“real estate”)on a carrier substrate and permits a greater number of dice to be placedon a substrate while providing a number of I/O's for each die well inexcess of that achievable using leadframes.

Various automation systems have been developed for accurate aligning andjoining the solder balls of an installed BGA to the contact sites of asubstrate. For example, U.S. Pat. No. 4,899,921 to Bendat et al.discloses a slender optical probe which is inserted between asemiconductor die and a substrate to be joined. Superimposed videoimages of the die and the substrate permit the two members to beaccurately aligned while they are narrowly separated. The probe isretracted and the two members brought together and joined.

In another system disclosed in U.S. Pat. No. 5,894,218 to Farnworth etal., an apparatus for aligning and positioning a die on a temporary testpackage utilizes video representations of the die surface and the testpackage to which the die is to be joined.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for fabricating areliable interconnect assembly comprising a BGA on a semiconductor die,larger semiconductor substrate, carrier substrate or other electroniccomponent workpiece.

More specifically, the present invention provides a method forextracting and disposing of single defective solder balls placed on orattached to attachment sites (e.g., pad or receptacle) of a BGA on asemiconductor die, larger semiconductor substrate, carrier substrate orother electronic component workpiece.

In this discussion, the term “defective ball” refers to a solder ballwhich has a high probability of causing difficulties in bonding to acarrier substrate, testing and subsequent use. Thus, a defective solderball includes one which is outside of the acceptable limits of size,shape, composition, surface finish and included contaminants, isinaccurately placed or misjoined to the attachment site on theworkpiece.

The present invention also provides a method for accurately placing anondefective solder ball on an attachment site vacated by a previouslyextracted, defective solder ball.

The method of the invention is applicable to any BGA, irrespective ofthe type of workpiece and the specific method by which the solder ballswere previously attached to the workpiece. In addition, the method isgenerally applicable to thermally softened solder balls of any size,shape, or composition useful in the semiconductor art.

In a method of the invention, a single-ball extractor/placer apparatuswith a corrosion-resistant capillary tube, heater, and optional vacuumsource may be used. A BGA formed on a workpiece is first scanned orotherwise tested to detect and identify the position of any defectivesolder balls. A capillary tube of the extractor/placer apparatus is thenpositioned over the defective solder ball and lowered to contact thesolder ball and apply heat to soften or melt the solder ball. In oneembodiment, capillary action alone may be employed to remove acompletely melted solder ball from the workpiece. Optionally, vacuumsuction may be applied to the solder ball through the capillary tubeand, as heat is applied to soften or melt the ball, assist in releasingit from the attachment site. In either instance, the capillary tube,with either the melted ball material therein or a softened defectiveball held thereon, is then lifted and moved to a location where the ballmaterial or ball is released, such as into a waste solder container.

The single-ball vacuum extractor/placer apparatus may then be used toaffix a nondefective replacement solder ball to the vacated attachmentsite. A fresh, nondefective solder ball is picked up by the capillarytube using a vacuum drawn therethrough. Flux is applied to the vacatedsite or to the solder ball held by the capillary tube, and the capillarytube lowered to place the ball in the desired site. Heat may be appliedby the heater to temporarily or permanently (through reflow) bond thereplacement solder ball to the workpiece. Alternatively, the reflow stepmay be accomplished in a furnace. The result is a workpiece in whichdefective solder balls are quickly and inexpensively replaced withoutwasting a large number of nondefective balls and without reprocessingthe entire workpiece.

An extractor/placer apparatus may be operated manually or may includevarious degrees of automation in the steps of identifying defectiveballs, as well as extracting and replacing solder balls identified asbeing defective.

Exemplary apparatus for performing the identification, extraction andplacement steps as disclosed herein are also contemplated as within thescope of the present invention, without limiting the present inventionto the examples described.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following drawings illustrate various features of the invention,wherein:

FIG. 1 is a schematic isometric view of an exemplary apparatus forextracting and replacing a defective solder ball from a ball grid arrayin accordance with a method of the invention;

FIG. 2 is a top view of a workpiece holder with a semiconductor diehaving a BGA thereon including defective solder balls to be extractedand replaced in accordance with a method of the invention;

FIG. 3 is a cross-sectional side view of a workpiece holder with asemiconductor die having a BGA thereon including defective solder ballsto be extracted and replaced in accordance with a method of theinvention, as taken along line 3—3 of FIG. 2;

FIG. 4 is a partially sectioned, partially cutaway side view of a ballpickup tool following extraction of a defective solder ball from a BGAon a workpiece in accordance with the method of the invention;

FIGS. 5 through 16 are schematic side views of a capillary tubeillustrating the steps relating to the extraction and replacement of asolder ball in a BGA in accordance with a method of the invention,wherein:

FIG. 5 illustrates the approach of a capillary tube to a defectivesolder ball;

FIG. 6 illustrates the seating of a defective solder ball on a capillarytube;

FIG. 7 illustrates the softening and extraction of a defective solderball by a capillary tube;

FIG. 8 illustrates the disposal of a defective solder ball;

FIG. 9 illustrates the attraction of a nondefective solder ball in aball reservoir by a capillary tube;

FIG. 10 illustrates the removal of a nondefective solder ball from aball reservoir by a capillary tube;

FIG. 11 illustrates the insertion of a nondefective solder ball into aflux reservoir;

FIG. 12 illustrates the removal of a nondefective solder ball withattached flux from a flux reservoir;

FIG. 13 illustrates the alignment of a fluxed solder ball with a vacatedattachment site of a BGA;

FIG. 14 illustrates the contact of a fluxed nondefective solder ballwith a vacated attachment site of a BGA;

FIG. 15 illustrates the thermal reflow of a fluxed nondefective solderball with a vacated attachment site of a BGA;

FIG. 16 illustrates the retraction of a capillary tube from the reworkedattachment site of a BGA;

FIG. 17 is a schematic isometric view of an exemplary automatedapparatus for extracting and replacing a defective solder ball from aball grid array in accordance with a method of the invention; and

FIG. 18 is a schematic chart characterizing control routes between acomputer controller and optical devices, positioners, switches and videooutput of an apparatus useful for extracting, removing and replacing asolder ball in accordance with a method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises an improved method and apparatus for reworking aBGA including one or more defective solder balls mounted on conductivesites on a surface of a workpiece. The workpiece may be, for example, asemiconductor die or a carrier substrate bearing circuit traces such asa printed circuit board. Usually, only a small fraction of semiconductordevices employing BGA's have defective solder balls, and very fewdevices have more than a few defective balls. The method of thisinvention permits easy extraction and replacement of only the defectiveballs, without removal of or adverse effects on the good (nondefective)solder balls.

More specifically, the invention pertains to a method for identifying,extracting and replacing a defective solder ball(s) of a BGA on aworkpiece. A defective solder ball is broadly defined as a solder ballwhich, unless corrected, will probably impair an operable, robustconnection through that solder ball to a conductive site when the BGA ofthe workpiece (semiconductor die or other substrate) is bonded,flip-chip style, to a corresponding pattern of conductive sites on acarrier substrate. Unless removed and replaced with a nondefectivesolder ball, the defective solder ball may result in short-circuiting,open-circuiting or other problems, rendering useless the assembly of thedie and the carrier substrate, as well other dice connected to thecarrier substrate, such as in the case of a multi-chip module. Thecurrent practice of removing all of the solder balls of a BGA when onlyone or a few are defective, cleaning the workpiece, and then replacingall of the solder balls is time-consuming, expensive, and wasteful ofmaterials.

In the method of the invention, each defective solder ball isindividually identified, extracted from the workpiece, disposed of, andreplaced by a fresh solder ball.

The method of the invention preferably utilizes an apparatus which willbe denoted herein as an extractor/placer apparatus 10.

As depicted in FIGS. 1, 2, 3 and 4, an exemplary manually operatedsolder ball extractor/placer apparatus 10 includes a base 12 and aworkpiece holder 30, such as a “precisor,” for retaining a workpiece 40during extraction of one or more defective solder balls 50A, 50B, 50C,etc., and replacement with good or nondefective solder balls 50. Theworkpiece holder 30 is shown as being attached to positioner 22 throughprecisor arm 64. Positioner 22 controls the movement of the workpieceholder in X-axis 14 and Y-axis 16. Positioner 22 includes Y-axispositioner 26 mounted on base 12, and X-axis positioner 24 mounted onthe Y-axis positioner. The X-axis positioner 24 is moved by rotation ofactuator knob 34, and the Y-axis positioner is moved by rotation ofactuator knob 36 for alignment of a ball attachment site 62 (see FIGS. 2and 3) on a workpiece 40 with vertical axis 32 through a pickup head 20described below. In FIGS. 2 and 3, a workpiece 40, typically asemiconductor device such as a chip-scale package, is mounted on aworkpiece holder 30. An arrangement or pattern comprising a BGA 70 ofsolder balls 50 is shown attached to attachment sites 62 on the activesurface 72 of the workpiece. Typically, the solder balls 50 areinstalled simultaneously with a multi-ball tool. The BGA 70 is depictedfor exemplary purposes as having three defective solder ballsrespectively identified by the numerals 50A, 50B and 50C. Ball 50Arepresents an undersized ball which will not make a connection duringbonding of the BGA 70 to the bonding sites of a substrate. Solder ball50B represents an unacceptably large or oversized ball, whose height mayprevent adjacent solder balls 50 from bonding with the substrate. Solderball 50C represents a ball which is out of alignment with the attachmentsite 62C and has run onto the active surface. During bonding with asubstrate, solder ball 50C may become undesirably electrically connectedto an adjacent solder ball 50, or it may not make acceptable contactwith the carrier substrate.

As indicated above and as shown in FIGS. 1 and 4, the extractor/placerapparatus 10 also includes a ball pickup tool or contact head 20 whichis attached through tool arm 28 to Z-axis positioner 68. The ball pickuphead 20 is controllably movable along vertical Z-axis 18 by rotation ofactuator knob 38 and may optionally be controllable in a rotationaldirection 48 about Z-axis 18. The pickup head 20 includes a hollowcapillary tube 42 having an inside diameter 43 which is less than anyball diameter to be encountered, enabling a solder ball 50 to beretained at the lower end 44 of the capillary tube 42. The lower end 44is configured as a seat for supporting a solder ball or other solderpreform 50 by suction force from a vacuum or negative pressure source 46(see FIG. 17). The capillary tube 42 is preferably formed of acorrosion-resistant material such as a stainless steel.

Also shown in FIG. 1 is a microscope 60 which is useful for inspecting aBGA on workpiece 40 to visually detect defective solder balls 50A, 50Bor 50C, for aligning the capillary tube 42 to a defective solder ball50A, B or C or to a vacant attachment site 62, and for determining theproximity of capillary tube 42 to a solder ball 50. The microscope 60may include an integral camera 61, not separately visible, bothsupported by support 66, and a closed circuit television display 98(CCTD) with memory (see FIG. 17) for use in inspecting the workpiece 40to identify and retain the locations of defective solder balls 50A, 50B,50C, etc. and to permit accurate positioning of the capillary tube 42relative to any defective solder ball 50A, 50B, 50C, and an attachmentsite 62. In one embodiment, the microscope 60 may be focused through thecapillary tube 42 to align a defective ball with the capillary tube axis32 and determine proximity of a ball 50 on workpiece 40 to the capillarytube 42.

As shown in FIG. 1, the exemplary extractor/placer apparatus 10 may alsoinclude a ball reservoir 80 for supplying a fresh solder ball 50 to thecapillary tube 42, a flux reservoir 82 into which a fresh solder ball 50may be dipped, and a defective ball receptacle 74 for receivingdefective solder balls extracted by the capillary tube. As shown, eachof the ball reservoir 80, flux reservoir 82 and receptacle 74 may bemovable to be more easily accessed by the capillary tube 42. Differentapparatus may be used for these purposes as known in the art.

The exemplary single-ball pickup or contact head 20 illustrated in FIG.4 comprises a body 54 attached to tool arm 28. A hollow capillary tube42 is attached to body 54 and may be interchangeable with capillarytubes of different sizes for use with different sizes of solder balls50. The interior 58 of the capillary tube 42 has a diameter 43 andcommunicates with internal chamber 56, fitting 85 and thereby to conduit86 leading to vacuum source 46 (see FIGS. 1 and 17). The lower end 44 ofthe capillary tube 42 may be configured as a partially spherical seat tomore securely hold a solder ball 50 under vacuum. As noted above, asolder ball 50A, 50B, or 50C may be melted by contact with capillarytube 42, and the molten solder material drawn by capillary action aloneor with vacuum assist into the interior or bore 58 to be subsequentlyexpelled therefrom as waste material. Alternatively, a gentle vacuum maybe applied through capillary tube 42 so that a softened or partiallymelted solder ball 50A, 50B, or 50C will be held at the end 44 of thecapillary tube 42, but molten solder in or on the ball will notgenerally be sucked through the capillary tube. The capillary tube 42 isformed of a corrosion-resistant, highly heat-conductive material such asstainless steel. A particulate filter 87 in filter housing 84 ispositioned to intercept bits of solder, flux, solder paste, dust, etc.,which may be sucked into the capillary tube 42 when vacuum source 46 isplaced into communication therewith.

A precision ground, clear glass window 52 provides access for a verticalmicroscope view into the chamber 56 and through the capillary tube 42 toa solder ball 50 or attachment site 62 below the tube end 44 and alongcapillary tube axis 32. Microscope 60 preferably includes an integral,focused light source 78 as known in the art, light source 78 being ableto illuminate workpiece 40, preferably through clear glass window 52 andaligned capillary tube 42 or, optionally, through optical fibers 53(shown in broken lines) extending from window 52 to locations adjacentcapillary tube 42 and focused adjacent the end 44 thereof. Anotherapproach is to illuminate workpiece 40 using a light source 55 (shown inbroken lines in FIG. 4) carried by tool arm 28.

Tool arm 28 and attached slider bar 67 are vertically movable relativeto Z-axis positioner 68. In addition, tool arm 28 is configured to bespring-biased so that when the capillary tube 42 is lowered onto asolder ball 50, the low resistance to compression of the spring will beabsorbed to prevent the ball from being unacceptably deformed. Thespring- biasing apparatus may be of any workable design, but is shown byway of example as a biasing spring 69 mounted between the tool arm 28and a slider bar 67 such that upward forces on the capillary tube 42will be absorbed and cushioned.

As illustrated in FIG. 4, a heater 90 surrounds a portion of capillarytube 42 to heat the latter to a temperature which will sufficientlysoften, e.g., melt or reflow, a solder ball 50. The heater 90 is shownas having electric resistance elements 91 and power cords 88 leading toa controllable power supply and/or temperature display output, not shownin FIG. 4.

In an alternative method of heating a solder ball to soften, reflow ormelt it, a laser device (not shown) may be oriented to direct a laserbeam along axis 32 through the interior 58 of the capillary tube 42, toproject on a solder ball 50 at the end 44 of the capillary tube 42.

In FIG. 4, an example of a workpiece 40 is shown with an array of solderballs 50 mounted on attachment sites 62. The workpiece 40 is mounted onworkpiece holder 30. A capillary tube 42 is depicted having extractedand lifted a defective solder ball 50C from an attachment site 62C.

FIGS. 5 through 16 illustrate exemplary steps used to perform theextraction and replacement of a defective solder ball 50A in a BGA of aworkpiece 40. FIGS. 5 through 16 presume that a defective solder ball50A is not reduced to a completely molten state and drawn into capillarytube 42. However, if a defective solder ball 50A is completely meltedand drawn into bore 58 of capillary tube 42, either by capillary actionor application of a vacuum, the steps illustrated by FIGS. 7 and 8 aremodified only in that the molten solder material is drawn into capillarytube 42 in FIG. 7 and then expelled therefrom into a defective ballreceptacle 74 under positive air pressure while still in a molten state.The steps illustrated by FIGS. 5, 6 and 9 though 16 remain the same.

In FIG. 5, a particular solder ball 50A has been identified as beingundersized. The central axis 32 of a capillary tube 42 of a ball pickuphead 20 is aligned with the ball 50A. With the vacuum on, the capillarytube 42 is lowered to place the capillary tube end 44 on the defectiveball 50A where the ball is attracted to the capillary tube end 44. Theheater 90 (see FIG. 4) is activated to soften (e.g., melt) and detachthe defective ball 50A from attachment site 62A as shown in FIG. 6. Inone embodiment, a slight upward biasing force may be applied to thecapillary tube 42 away from the attachment site 62A to facilitaterelease of a heat-softened solder ball 50A from the attachment site 62A,the capillary tube 42 and vacuum-held softened solder ball 50A thenretracting slightly from attachment site 62A. Such a retractionmechanism may be in the form of a spring-loaded solenoid of whichcapillary tube 42 forms a part, the solenoid being powered to overcomethe retractive force of the spring until retraction of a solder ball 50Ais desired. Alternatively, a spring may be used to extend the capillarytube 42 until it is desired to retract the same when the solenoid ispowered.

FIG. 7 depicts the defective solder ball 50A being lifted andtransported to a disposal site, such as defective ball receptacle 74,shown in FIG. 8. The defective solder ball 50A is discarded byinterrupting the vacuum provided to capillary tube 42. Another option,not shown, is to reverse the pressure in capillary tube 42 and use abrief puff of air from a positive pressure source to dislodge the solderball 50A.

A nondefective (i.e., “good”) solder ball 50 is then attracted by vacuumin the capillary tube 42 from a solder ball reservoir 80 (FIG. 9) andpicked up from the reservoir (FIG. 10).

As depicted in FIG. 11, the capillary tube 42 then submerses a portionof the carried good solder ball 50 into flux 83 within a flux reservoir82. When the good solder ball 50 is lifted from the flux reservoir 82(FIG. 12), a coating of flux 83 (exaggerated for clarity) will cover aportion of the good solder ball 50, enabling effective attachment of thegood solder ball 50 to a vacated attachment site 62A. Alternatively,flux may be placed on the vacated attachment site 62A prior to placementof good solder ball 50 thereon.

The fluxed, good solder ball 50 is then aligned and positioned directlyover the vacated attachment site 62A, as shown in FIG. 13. The capillarytube 42 is then lowered to place the good, fluxed solder ball 50 on theattachment site 62A (FIG. 14) without placing undue compressive force onthe good, fluxed solder ball 50. The biasing spring 69 serves to limitany force on the good solder ball 50 due to inadvertent overrun of thecapillary tube end 44. The capillary tube 42 is heated to soften, oroptionally reflow or completely melt the solder ball 50, joining it tothe attachment site 62A (FIG. 15), and the capillary tube 42 isretracted (FIG. 16) to seek another defective solder ball 50 (forexample, oversized solder ball 50B) or for use with another workpiece40.

The process of locating, extracting and replacing defective solder balls50A, 50B, 50C, etc., on a workpiece 40 may be automated to effectivelyaccelerate the process. Various steps of the method may be automated,and an example of such method and apparatus follows.

As shown in FIG. 17, an extractor/placer apparatus 10A includes featuresof the apparatus 10 depicted in FIG. 1 and described with respectthereto, with certain additional automated features. The variouscomponents of the apparatus 10A are controlled by a computer controlboard 92 including at least one microprocessor, in conjunction withappropriate programs and data in associated memory storage 94. A videodisplay 98 provides an enlarged visual output which enables easydetection of defective solder balls 50A, 50B, 50C, etc., and precisealignment of the capillary tube 42 of the ball pickup head 20.

FIG. 18 schematically depicts the control lines 110 linking the controlboard 92, memory 94, keyboard 96, video 98 and the various controlledcomponents of apparatus 10A.

A workpiece 40 is mounted in a workpiece carrier or holder 30. Theholder 30 is mounted on a workpiece positioner 22 comprising an X-axispositioner 24 and a Y-axis positioner 26, both of which are controlledby integral electric drive assemblies which may comprise, for example,linear steppers or fine-pitch screw drives. The positioner 22 iscontrolled by a program stored in memory in the control board 92 and/orin memory storage 94.

A microscope 60 with an associated camera 100 is positionally controlledfrom control board 92. Although defective solder balls may have beenfound in a previous inspection operation, the microscope 60 may bemanually used with eyepieces 76 to verify solder ball faults. However, avideo display 98 of the output of camera 100 makes the task much easier.Automated analysis using conventional “machine vision” methods may beemployed in a comparison of real-time video or still photographs of theactive surface of workpiece 40 with a stored digital image of a modelBGA 70 to detect unacceptable variations in size and placement of solderballs 50. Digital imaging may also be employed to rapidly compareindividual solder balls 50 of workpiece 40 with design specifications orfor coincidence with visually perceptible defects, images of which arestored in memory storage 94. An auxiliary camera 100A may be positionedto view the workpiece 40 from an oblique angle 114, i.e., along slopingline 112. Combining the views from both cameras may enable more accuratedetermination of defective solder balls 50A, 50B, 50C, etc. Of course,more than one auxiliary camera may be employed to enable multipleperspective views. Microscope 60 with camera 100 is shown as laterallyextendable in direction 104 and horizontally rotatable in direction 106about a vertical axis. In addition, the microscope/camera combinationmay be rotated about a horizontal axis as shown by arrow 108. It may benoted that in a fully automated system, a camera without visualmicroscope capability may be employed as desired.

A self-contained machine vision system available from a commercialvendor of such equipment may be employed. For example, and withoutlimitation, such systems are available from Cognex Corporation ofNatick, Mass. For example, the apparatus of the Cognex BGA InspectionPackage™ or the SMD Placement Guidance Package™ may be adapted to thepresent invention, although it is believed that the MVS-8000™ productfamily and the Checkpoint® product line, the latter employed incombination with Cognex PatMax™ software, may be especially suitable foruse in the present invention.

It is noted that a variety of machine vision systems are in existence,examples of which and their various structures and uses are described,without limitation, in U.S. Pat. Nos. 4,526,646; 4,543,659; 4,736,437;4,899,921; 5,059,559; 5,113,565; 5,145,099; 5,238,174; 5,463,227;5,288,698; 5,471,310; 5,506,684; 5,516,023; 5,516,026; and 5,644,245.The disclosure of each of the immediately foregoing patents is herebyincorporated by this reference.

Vertical movement of the single-ball pickup head 20 through Z-axispositioner is controlled from the control board 92. The Z-axispositioner may also be controllably rotatable about a vertical axis asshown in FIG. 1. The capillary tube heater 90 is controllable (ON/OFF)from the control board 92, as is an ON/OFF valve 102 controlling vacuumfrom a vacuum source 46. In addition, movements of the defective ballreceptacle 74, ball reservoir 80 and flux reservoir 82 are allcontrollable from the control board 92. Thus, precise positioning of thecapillary tube's end 44 above any of defective balls 50A, 50B, 50C, anattachment site 62, a defective ball receptacle 74, a good ballreservoir 80, and a flux reservoir 82 is achieved.

The method of the invention may be used simply to remove solder ballsfrom a BGA for any purpose, whether the ball is defective or good butmislocated or extraneous.

It should be noted that various terms are used herein in their broadsense. For example, the term “flux” refers to any substance used inconjunction with a solder ball at the time it is reflowed. The term“solder ball” includes not only preformed balls of solder but alsosolder paste pellets or other preforms used to replace a defectivesolder ball 50.

Use of this method results in rapid and accurate rework of a BGA 70having defective solder balls 50A, 50B, 50C, etc. The inventive methodalso avoids the wastage of a large number of good solder balls 50 whichoccurs when an entire BGA is removed. It avoids excess use of flux fromrepeated attachment of the solder balls of an entire BGA. The methodalso avoids repeated heating of the workpiece to reflow an entire BGAfor removal and then again for ball replacement, which operations may,if the heat budget for a device is exceeded, result in irreparabledamage.

The methods and apparatus described herein present many advantages inreworking a BGA workpiece, including higher reliability, lower cost,reduced ball wastagei etc. As used herein, the term “BGA” means andincludes any array or pattern comprising a plurality of solder balls onan electronic component workpiece and is not limited to an arraycomprising rows and columns of balls or any other specific pattern.

The embodiments of the invention as described herein are intended to beillustrative and not restrictive, and the scope of the invention isdefined by the appended claims rather than the preceding description.Those of ordinary skill in the art will recognize and appreciate thatadditions, deletions and modifications to the disclosed embodiments, andcombinations of features from different embodiments, are possible andeasily effected without departing from the scope of the invention. Allvariations that fall within the metes and bounds of the subject matterclaimed, or which are equivalent thereto, are, therefore, intended to beembraced by the following claims.

What is claimed is:
 1. A method for correcting a defective ball gridarray on an electronic component workpiece, the method comprising:disposing at a first location a workpiece having an array of solderballs secured thereto; and without moving the workpiece from the firstlocation: identifying at least one defective solder ball associated witha corresponding attachment site of the array of solder balls on theworkpiece; and extracting the at least one defective solder ball with acapillary tube to vacate the corresponding attachment site byindividually heating the at least one defective solder ball to a moltenstate and removing molten solder from the corresponding attachment siteinto the capillary tube by capillary action; and replacing the at leastone defective solder ball with a nondefective solder ball placed on thevacated corresponding attachment site with the capillary tube whilemaintaining other solder balls of the array of solder balls inundisturbed arrayed attachment to the workpiece.
 2. The method of claim1, wherein the at least one defective solder ball comprises a pluralityof defective solder balls, and further comprising extracting andreplacing each defective solder ball of the plurality of defectivesolder balls while maintaining said other solder balls of the ball gridarray in said undisturbed arrayed attachment to the workpiece.
 3. Themethod of claim 1, further comprising assisting the capillary action byapplication of a vacuum.
 4. The method of claim 1, further comprisinglifting the nondefective solder ball, positioning the nondefectivesolder ball on the vacated corresponding attachment site and locallyheating only the nondefective solder ball at least to a temperature tocause the nondefective solder ball to bond to the correspondingattachment site.
 5. The method of claim 4, further comprising liftingthe nondefective solder ball using a vacuum provided through a capillarytube, and locally heating the nondefective solder ball through thecapillary tube.
 6. The method of claim 4, further comprising applyingflux to the nondefective solder ball prior to the positioning thereof onthe vacated corresponding attachment site.
 7. The method of claim 4,further comprising applying flux to the vacated corresponding attachmentsite prior to the positioning of the nondefective solder ball thereon.8. A method for removing and replacing a solder ball from an attachmentsite of a ball grid array on an electronic component workpiece, themethod comprising: placing an electronic component workpiece having aball grid array thereon at a location; and without moving the electroniccomponent workpiece from the location: aligning a capillary tube over asolder ball of the ball grid array on the electronic componentworkpiece; lowering the capillary tube to contact the solder ball fromabove; heating the solder ball with heat conducted from a heat sourcethrough the capillary tube sufficiently to reduce the solder thereof toa molten state; drawing a vacuum through the capillary tube tofacilitate drawing the solder in a molten state thereinto; retrieving areplacement solder ball from a source of replacement solder balls usingthe capillary tube; aligning the capillary tube over the attachmentsite; lowering the capillary tube to place the replacement solder ballin contact with the attachment site; heating the replacement solder ballthrough the capillary tube at least sufficiently to cause thereplacement solder ball to bond to the attachment site; and retractingthe capillary tube and releasing the replacement solder ball.
 9. Amethod for extracting and removing at least one defective solder ballfrom an attachment site in a ball grid array of an electronic componentworkpiece, comprising: placing an electronic component workpiece on asupport at a location; and without moving the electronic componentworkpiece from the location: viewing a ball grid array of the electroniccomponent workpiece under magnification to identify and locate at leastone defective solder ball in the ball grid array; positioning a ballpickup head comprising a capillary tube over the at least one defectivesolder ball and vertically extending the capillary tube to contact theat least one defective solder ball; applying heat from a heater throughthe capillary tube to the at least one defective solder ball to reduceit to a molten state; removing the molten solder from the attachmentsite with the capillary tube by at least one of capillary action andapplying a vacuum to the molten solder material through the capillarytube; retrieving a good solder ball from a source of good solder ballsusing the capillary tube; positioning the capillary tube over theattachment site; extending the capillary tube to place the good solderball in contact with the attachment site; heating the good solder ballwith the heater through the capillary tube at least sufficiently tocause the good solder ball to bond to the attachment site; andretracting the capillary tube to release the solder ball.
 10. The methodof claim 9, further comprising, after removing the molten solder fromthe attachment site with the capillary tube: retrieving the good solderball from the source of good solder balls with the capillary tube byapplying a vacuum through the capillary tube; and applying flux to atleast one of the one good solder ball and the attachment site.
 11. Themethod of claim 9, further comprising performing at least some of themethod for extracting and removing at least one defective solder ballunder control of a programmed microprocessor.
 12. The method of claim11, wherein viewing the ball grid array under magnification is performedusing a machine vision system.
 13. The method of claim 12, furtherincluding using the machine vision system in combination with saidprogram microprocessor and data stored in memory to compare the viewedball grid array with a model ball grid array and said location of the atleast one defective solder ball in the viewed ball grid array.
 14. Anapparatus for extracting and replacing an individual solder ball mountedon an attachment site on an electronic component workpiece, theapparatus comprising: a support configured to receive an electroniccomponent workpiece thereon; a solder ball contact head configured toremove an individual solder ball from the electronic component workpieceand, without removal of the electronic component workpiece from thesupport, to replace the removed solder ball with another solder ball,the solder ball contact head comprising: a capillary tube including adistal end having an inside diameter less than a diameter of anindividual solder ball of a plurality of solder balls mounted in anarray over a surface of the electronic component workpiece; and aheating device operably coupled to the capillary tube; a structure foraligning the capillary tube with the mounted individual solder balls insubstantially transverse orientation to the electronic componentworkpiece surface; and an assembly for controllably extending andretracting the capillary tube toward and away from the surface of theelectronic component workpiece.
 15. The apparatus of claim 14, furtherincluding a vacuum source selectively operably coupled to the capillarytube for drawing a vacuum therethrough.
 16. The apparatus of claim 14,further including a structure for effecting alignment of the support inX- and Y-directions.
 17. The apparatus of claim 14, further including aviewing apparatus configured for magnification and orientable over thesurface of the electronic component workpiece when the electroniccomponent workpiece is received on the support.
 18. The apparatus ofclaim 17, wherein the viewing apparatus includes at least one of atleast one eyepiece for viewing and a camera for generating an electronicimage.
 19. The apparatus of claim 17, further including a light sourceorientable for illuminating the surface of the electronic componentworkpiece.
 20. The apparatus of claim 19, wherein the solder ballcontact head is configured for transmission of at least a portion of theillumination from the light source therethrough.
 21. The apparatus ofclaim 20, wherein the solder ball contact head is configured with awindow in alignment with a bore extending through the capillary tube.22. The apparatus of claim 20, wherein the solder ball contact head isconfigured with at least one optical fiber positioned to receive lightfrom the light source and extending to a location to transmit thereceived light onto the surface of the electronic component workpiece.23. The apparatus of claim 14, further comprising a source ofnondefective solder balls in proximity to the solder ball contact head.24. The apparatus of claim 14, wherein said structure for aligning thecapillary tube comprises a machine vision apparatus including a camera,a magnifying lens, at least one programmed microprocessor and memoryhaving data stored therein representative of the surface of theelectronic component workpiece and topography thereof.
 25. The apparatusof claim 24, wherein the memory further includes stored data usable bythe at least one programmed microprocessor for identifying defectivesolder balls within the array.
 26. The apparatus of claim 25, whereinthe at least one programmed microprocessor is programmed to control atleast some movements of the solder ball contact head and operation ofthe heating device.
 27. The apparatus of claim 14, wherein the solderball contact head is resiliently biased to yield responsive to contactof the capillary tube with a solder ball of the array.
 28. The apparatusof claim 14, further including a retraction device for selectivelyretracting the capillary tube toward the solder ball contact head.